The present disclosure relates generally to neutron well logging and, more particularly, to a neutron porosity downhole tool employing a neutron monitor for improved precision and reduced lithology effects.
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Downhole tools for neutron well logging have been used in oilfield settings for many years to measure formation porosity and as gas and lithology indicators. These downhole tools have historically included a radioisotopic neutron source, such as AmBe, which emits neutrons into the surrounding formation. The neutrons may interact with the formation before being subsequently detected in neutron count rates by one or more neutron detectors. Among other things, the neutron count rates may be sensitive to hydrogen in formation pore spaces. In general, the more hydrogen there is in the formation, the fewer neutrons arrive at the detector. Since formation porosity is generally water or hydrocarbon-filled, the neutron count rates may be employed to determine a porosity of the formation.
When a radioisotopic source is employed by a downhole neutron porosity tool, porosity may be determined based on the detector count rate normalized to the neutron output of the source, which may be predictable. Indeed, it may be sufficient to perform a one-time calibration to determine the neutron output of the radioisotopic source if it has a sufficiently long half life as in the case of 241AmBe. Thereafter, any change in the future can, in principle, be predicted from the known half-life of the radioisotopic material. However, a radioisotopic neutron source may be undesirable for a variety of reasons. For example, the use of a radioisotopic source may involve negotiating burdensome regulations and the sources may have limited useful lives (e.g., 1 to 15 years). Moreover, radioisotopic sources are becoming more expensive and more difficult to obtain.
Alternative neutron sources, such as electronic neutron generators, may be used in place of a radioisotopic neutron source in a neutron porosity tool. However, in contrast to the predictable output of a radioisotopic neutron source, the output of an electronic neutron generator may be difficult or impossible to predict from the operating parameters of the tool. For this reason, neutron generator-based devices may generally determine porosity from a ratio of count rates from detectors at different spacing (e.g., a near/far count rate ratio) in order to cancel out any variations in the output of the neutron source. While this method may achieve its stated goal and may introduce certain other positive effects (e.g., reducing the device's sensitivity to several borehole effects unrelated to porosity), it also may reduce the porosity sensitivity of the tool, since some of the individual detectors' sensitivity also may be canceled out in the ratio. This reduction in sensitivity may be especially problematic when using the deuterium-tritium (d-T) reaction-based neutron generators generally employed in the oilfield. These generators produce 14 MeV neutrons, more than twice the average energy of neutrons from an AmBe source. Used in a device with typical near and far source-detector spacings of around 1 and 2 feet, respectively, this higher neutron source energy results in a dramatic drop in porosity sensitivity at high porosity compared to an AmBe-based device.
To mitigate the above concerns, certain neutron porosity tools that would measure porosity based on a single neutron count rate relative to the monitored output of such an electronic neutron source might have been contemplated in the past. However, at the time such tools would have been considered, they would have produced porosity values with excessive environmental effects. To compensate for these environmental effects, some sources suggested increasing the spacing between the neutron source and the neutron detector in such a neutron porosity tool. While doing so could reduce the environmental effects of measurements in the contemplated tools, increasing the source-detector spacing has been shown to increase the lithology effect.